In a signal transmission/reception device such as an optical transceiver, generally, diagnosis has been performed as to whether or not a signal transmitting/receiving operation is being normally performed, thereby to enhance reliability of the operation. The diagnosis function is realized by a signal monitoring device provided inside or outside the signal transmission/reception device.
As a technique for the above diagnosis, for example, there is one where in an optical transceiver or the like, an analog signal which is a parameter concerning operation characteristics or an analog signal which is a parameter concerning an environment is converted to a digital signal, to regularly monitor the digital signal, and when it exceeds a predetermined threshold, a predetermined flag bit is generated (cf. Patent Document 1). The flag bit is information complying with the XFP standard, for example. When a request is made from an external host-side device, the presence or absence of the flag bit is reported, to allow a diagnosis result to be grasped.
Further, for example, there is one where in an optical transceiver or the like, a transmitted/received signal is regularly monitored, and a maximum value and a minimum value of the signal values are stored (cf. Patent Document 2). The optical transceiver references the maximum value and the minimum value, to perform comparison processing with a predetermined threshold, and it is thereby possible to judge whether or not the signal value has exceeded the predetermined threshold.